Elastomeric compounds are so-called viscoelastic materials. This means that the properties which they exhibit depend on the duration (time or frequency) and on the temperature at which external stresses or deformations (strains) are applied to them.
The balance and level of such viscoelastic properties determine the processibility and the range of end-use characteristics of these elastomeric compounds, and, therefore, their practical applications. With present technologies, a wide range of applications is possible due to the fact that the basic constituents of these elastomeric compounds, namely, naturally-occurring or synthetically-produced rubbers, can be mixed or compounded with numerous chemicals and other additives, so as to tailor and customize their viscoelastic properties. The technology of developing vulcanizable elastomeric compounds for specific applications has achieved a high degree of sophistication over the past years.
As a result of developments in electronics and dynamic test equipment, great advances have also been made in the precise measurement of viscoelastic properties of elastomeric compounds, and also in the correlation of such measurements with the performance of these compounds in engineered products, such as tires.
Pertinent viscoelastic parameters, which are measured in laboratory tests, are the elastic moduli, E′ (in tension or compression) and G′ (in shear), the viscous moduli E″ (in tension or compression) and G″ (in shear), and the ratio of the viscous and elastic moduli, otherwise known as the loss tangent or tan δ. These parameters are determined under dynamic conditions at specific temperatures, frequencies, stain rates, and stress- or stain-amplitudes. Another important parameter is the glass transition temperature, Tg, which is the temperature below which the elastomeric composition becomes “glass-like” or brittle.
With present testing technology, these viscoelastic parameters can be determined with great precision, and these parameters can be confidently correlated to practical performance characteristics in, say, tires. For instance, with tire tread compounds, the tan δ of a compound, measured within a temperature range of about 50-70° C., correlates directly with the rolling resistance of a tire. That is, the lower the tan δ the lower the rolling resistance of a tire tread. Similarly, the magnitude of the tan δ or E′, measured at about 0° C., or at the respective Tg of a tire tread compound, relate to certain traction characteristics of a tire, whereas the magnitude of the tan δ at very low temperatures of about −65° C. is indicative of the abrasion or wear characteristics of a tire tread. Regarding traction and wear, the greater the values of the respective viscoelastic parameters, the better the performance of the compounds in tire treads.
To be specific, with respect to predicting the potential rolling resistance of tire tread compounds, differences in tan δ of 0.005 are significant and beyond experimental error, while changes of 0.015 and greater are significant with respect to certain traction characteristics and can be observed in actual tire performance tests.
In practice, there are opposing performance trends of elastomeric compounds, which usually requires compromises when optimizing their viscoelastic properties. For instance, improvements in the tan δ leading to a lower rolling resistance of tire tread compounds we generally also accompanied by a reduction of tan δ at other relevant temperatures, thus resulting in a potentially lower wet traction performance of the tire tread. Similarly, there are generally also opposing trends with respect to certain properties, such as traction capabilities of tire tread compounds and their abrasion or wear resistance characteristics.
Much effort has been spent on developing compounding technologies and new compound additives to ameliorate this problem of opposing property trends while raising the overall performance levels. Great progress has been made through what is now commonly referred to as “silica compounding” technology, but there is still need for further technical improvements. The present invention demonstrates that this is now possible.